(a) Field of the invention:
This invention relates to microscopes and, more particularly, to a microscope provided with an automatic focusing device.
(b) Description of the prior art:
As automatic focusing devices for microscopes, a system wherein focusing is made by measuring the distance between an objective and a sample surface with a laser light or the like. Another system wherein focusing is made by measuring the distance by pressure variation of air jetted toward the sample surface from the tip portion of the objective. However, in these systems, there are defects which exist. Particularly in the case of a microscope for living objects, the in-focus position will be shifted, for example, by the change or the like of the thickness of the cover glass. Therefore, accurate focusing will be difficult and a change from the difference of the focus of the objective will be also a problem.
Therefore, a microscope of a system wherein an image pickup device is used to attempt to overcome these problems. However, in using this system, focusing will be possible only when the conditions are set by a complicated microscope operation and then the conditions required for the image pickup device are met. Therefore, it has been difficult to use as an automatic focusing system. That is to say, there is a problem that, in the image pickup device, due to its characteristics, the amount of the light projected as an optical image and the sensitivity (electric charge accumulating time) of the image pickup device itself must be properly set. The amount of the light projected onto the image pickup device varies with the amount of the light from the light source, numerical aperture (NA value) of the objective to be used, state of the sample, switchover of the ND filter and stop value of the aperture stop. Therefore, a method wherein, in order to control the electric charge accumulating time and the like of the image pickup device, the image signal is fed back to the driving circuit of the image pickup device and is again obtained has been proposed. However, this method has defects that the number of times of the data input is high, the processing time is long, the dynamic range of the image pickup device can not be well used and the S/N ratio of the image signal varies with the amount of the incident light.
When photographing by using an objective of a low magnification, the depth of field on the sample side will be so deep that, if focusing is made by observing through an eyepiece, wherever the focal point may be, it will be observed as if in-focused by the adjusting action of the eye but, on the other hand, the focal depth will be so shallow on the image side that, on the film surface, no good focusing will be able to be made.